Place unsalted butter in a medium saucepan over medium heat. Allow the butter to melt, then continue cooking for an additional 5-10 minutes. You should see the butter begin to foam vigorously – keep stirring.
Add brown sugar, then whisk until well combined. Set aside to cool for 5 minutes.
In a separate bowl, whisk together all-purpose flour, kosher salt, ground cinnamon, baking powder, and ground nutmeg.
Add large eggs, whisking after each addition for about 10 strokes, then add vanilla extract and stir to combine.
Pour the blondie batter into the prepared pan, then use an offset spatula to smooth the top into a flat even layer. Bake until the top is light golden brown. Cool in the pan on a wire rack.
Drizzle all over the top of the cooled blondies, covering the entire surface. If you want to sprinkle extra chopped nuts on top, now is the time to do so. Allow to harden.
Once the glaze is set, transfer the blondies from the pan and cut.